The catalytic hydrogenation of nitrobenzene in acidic solutions is generally well known. The process is believed to involve the formation of an intermediate product .beta.-phenylhydroxylamine which is rearranged to form p-aminophenol and aniline. Illustrative of the processes for preparing p-aminophenol are U.S. Pat. Nos. 2,198,249, 2,765,342, 3,079,435, 3,265,735 and 3,383,416.
While sulfides of certain heavy metals such as cobalt, molybdenum, and tungsten have been proposed as hydrogenation catalysts, platinum and platinum on carbon and/or alumina are conventionally used in the conversion of nitrobenzene to p-aminophenol. While the conventional platinum catalysts are well suited for the preparation of commercially significant quantities of p-aminophenol, they are capable of further hydrogenating the p-aminophenol to alicyclic compounds which are undesirable by-products. This is particularly the case where hydrogenation takes place in the presence of a quantity of platinum catalysts.
Further, the nitrobenzene cannot be hydrogenated to completion without overhydrogenation by use of platinum catalysts. Thus, in the conventional hydrogenation reaction, the process must be stopped prior to completion to avoid the formation of undesired alicyclic compounds. This requires the additional step of recovering the unconsumed nitrobenzene by steam distillation.
Moreover, conventional platinum catalysts are easily poisoned and are not reusable unless returned to a precious metal refiner to recover the metal.